Air cycle environmental control systems are typically employed to cool, dehumidify, pressurize, and otherwise condition air in aircraft passenger cabins. In these systems, supply air, which is ambient air compressed by either the engine or the auxiliary power unit, is delivered to a compressor. The temperature of the compressor outlet air is then lowered in a ram-air cooled heat exchanger before it is expanded to cabin pressure in a turbine, not only producing the chilled air needed to satisfy aircraft cooling requirements, but generating the power used to drive the compressor as well.
As the aircraft ascends and ambient air pressure decreases, since the compression ratio of the supply air source is relatively constant, the pressure of the supply air also falls. When the pressure of the supply air entering the compressor falls, the pressure at the turbine inlet drops as well. Since the pressure at the turbine outlet, cabin pressure, varies only slightly with altitude, when the pressure at the turbine inlet drops, less power is extracted by the turbine. With less power available to further compress supply air, turbine inlet pressure decreases even more. At every ambient air pressure, therefore, there is a system equilibrium point at which the amount of power absorbed by the compressor equals the amount of power extracted by expanding the air exiting the compressor to cabin pressure.
As the power level of the system fluctuates in response to changing ambient air pressure, the pressure and flow rate through the system change as well. The compressor and turbine are therefore chosen such that, at some baseline altitude, the temperature and mass flow rate of the air exiting the turbine meet or exceed some predetermined cooling capacity levels. As cooling requirements are typically most stringent when the aircraft is on the ground, in most cases this baseline altitude is selected to be at or near sea-level altitude. At low altitudes, then, the environmental control system satisfies all aircraft cooling needs. At higher altitudes, however, where the system operates at pressures below the design pressure, the mass flow rate of air exiting the system, and hence its cooling capacity, is decreased. With some system configurations, cooling capacity at these altitudes can be insufficient to adequately cool the aircraft. With this approach, therefore, to satisfy cooling requirements at low altitudes, cooling capacity is sacrificed at the high cruise altitudes encountered during the majority of the flight.
To improve cooling capacity during high altitude flight, the compressor and turbine can instead be selected to ensure that the system satisfies aircraft cooling requirements during low pressure operation instead of high. Adequate cooling is then realized during a higher percentage of the flight time, since low altitude conditions are encountered only during take off and landing maneuvers. During these maneuvers, however, the supply air pressure rises, exceeding the system design pressure. Since the flow area of the system is fixed, the flow rate through the system increases excessively. To avoid this, some mechanism must be included to limit supply pressure. If the supply air pressure is limited, heat exchanger performance, and hence size, must be increased to compensate for the decline in turbine pressure ratio and resultant temperature drop. With this approach, there must then be a compromise between excessive flow at low altitudes and system size.
To improve system flow rate, and hence cooling capacity, when the pressure of the ambient air falls at high operation altitudes, some systems employ turbines with two sets of inlet nozzles. The flow area of the nozzle on a turbine is typically fixed. In these dual-nozzle turbine configurations, however, air is delivered, through either two separate scrolls or a single divided scroll, to two sets of alternating nozzles, one having a larger flow area than the other. To alter the flow area of the system during flight, a valve in the scroll supplying air to the smaller nozzle set is opened or closed. The compressor and turbine in dual-nozzle systems are chosen such that the system cooling capacity at sea-level altitude meets or exceeds, when the valve in the smaller nozzle scroll is closed, cooling requirements. When the aircraft ascends to higher altitudes, the valve in the smaller nozzle scroll opens, allowing an increased mass flow rate of air to pass through the system, improving high altitude cooling capacity. However, as the flow characteristics of the turbine change, so does the efficiency of the compressor. When the valve in the scroll opens, compressor efficiency falls. In this configuration, then, although overall cooling capacity increases when the scroll valve opens, disproportionately more supply pressure is needed to generate that cooling flow.
The compressor and turbine in a dual-nozzle system can also be sized for high-altitude operation with the scroll valve open, instead of low-altitude operation with the scroll valve closed. A surge problem, however, results when this technique is used. As the aircraft descends, the pressure of the supply air, and hence the flow through the compressor, increases. When the pressure of the supply air rises above a predetermined level, the scroll valve closes. Not only, then, does the compressor pressure ratio increase at lower altitudes, but, since the system flow area is reduced, the flow rate through the system decreases as well. At this point, therefore, the flow rate through the compressor is too low, given the pressure ratio, to prevent air flowing over the compressor blades from stalling, and the compressor surges.